I love this description: "Also it was now undoubtedly hot in the tunnel. Wisps of vapour floated up and past him and he began to sweat. A sound, too, began to throb in his ears, a sort of bubbling like the noise of a large pot galloping on the fire, mixed with a rumble as of a gigantic tom-cat purring. This grew to the unmistakable gurgling noise of some vast animal snoring in its sleep..."

I can imagine Smaug with certain feline characteristics, such as long-term naps with an occasional stretching of the appendages, and rare moments of wakefulness during which he tracks and pounces on prey. I wonder if he grooms himself the way a cat would...?

That bubbling sound is his fire-producing mechanism at work, a constant simmering in his throat, and the exhaled moisture from his lungs, as it passes through that internal cauldron, becomes the heated vapour which greets Bilbo on his way down the passage. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~